A major limitation to the design of compact, high-power density synchronous electric motors and generators is the ability to remove heat from the rotor field windings and rotor surface to maintain suitably low component temperatures. Improvements in the efficiency of cooling rotating electrical machinery result in increased capacity or power output for a given machine. With machines which are cooled by the circulation of air or other gases, the coolant is generally forced through various passages in the machine by a fan. Attempts to improve cooling efficiency by maximizing the exposure of machine components to coolant and minimizing obstructions to the flow of the coolant have resulted in machine designs which include coolant passages throughout the rotors and stators of synchronous electric machines.
The rotors of synchronous electric machines, either motors or generators, experience ohmic heating within the field windings due to the flow of dc excitation current, and on the outer surface of the rotor by the flow of eddy currents induced by armature currents and relative motion. The useful operating capability of such machines is limited by the ability of rotor cooling systems to remove this heat while maintaining component temperatures at levels acceptable for insulation life and mechanical stability of the rotor. In general, advances in machine rating per unit of volume or weight have been achieved by increase of the effectiveness of rotor and stator cooling.
During normal operation of synchronous machines, heat is generated in the various pole pieces mounted on the peripheral surface of the rotor; and if this heat is not transferred away from these pole pieces, serious reduction in the efficiency and power output of the machine results as discussed herein above. Many different arrangements have been resorted to in the past in an attempt to maintain the pole pieces at as low a temperature as possible in order to achieve an optimum efficiency of the device.
Machines which utilize salient pole rotors have been provided with wedges in the interpole spaces to conduct coolant through these spaces. Such a device is disclosed in U.S. Pat. No. 2,899,573 issued to Wesolowski. This device discloses a salient pole rotor cooling wedge for maintaining the position of the rotor field windings which permits the flow of air into and through the wedges. In doing so, the continuous flow of air through the wedge is used to transfer heat from the rotor field windings to the atmosphere.
Similar to the foregoing, U.S. Pat. No. 4,409,502 issued to McCabria and assigned to the assignee of the subject application discloses a self-cooled rotating electrical machine which is provided with multiple axial and radial coolant passages. A-shaped wedges are positioned in each interpole space with the wedges including a vane and impeller on each end to force cooling air through the machine. Air entering through the ports in the frame assembly passes through an exciter generator which is located at the drive end of the frame assembly. Air entering through other ports in the frame assembly pass through a rotating rectifier assembly located within the rotor. After passing through the excited generator and the rotating rectifier assembly, the air enters the main cooling circuit of the machine. Each of the A-shaped wedges include an axial rotor coolant passage which runs the length of the rotor. As the rotor turns vanes on the ends of the wedges impart an outward flow due to centrifugal force action upon the air. Part of the air enters the slots in the wedges at the anti-drive end of the rotor. A portion of that air is expelled at the center of the rotor through holes in the center of the wedges while the remainder is expelled at the drive end of the rotor. By utilizing the hollow A-shaped wedges, air is allowed to pass through the wedges and transfer heat from the rotor field windings to the surroundings, thus cooling the windings. However, by using air flow to cool the space within the wedges, the rotor will experience drag which may be detrimental to the efficiency of the electrical machine.
An alternative to the aforementioned use of air flow to cool the rotor field windings of synchronous electric motors and generators, is the use of a rotating heat pipe as disclosed in U.S. Pat. No. 3,715,610 issued to Brinkman. Therein, heat which is generated within a rotor is removed utilizing a heat pipe rotating about the rotor shaft. Heat is absorbed from the rotor by vaporization with a portion of the refrigerant within a rectangular evaporator juxtaposed with the heat generating region. The evaporator is positioned within the rotor coils; e.g., at the center of the field pole winding of a synchronous machine in order to conduct heat away from the field pole windings. The configuration of this heat pipe involves an external condenser constructed of finned tubing. Support of such a configuration in a high speed rotor is impractical. Also, with such a cooling device, any maintenance of the heat pipe would, in most cases, require the entire disassembly of the rotor including that of the windings in order to gain access to the heat pipe. This configuration of heat pipe also utilizes space which would be otherwise available for field winding copper. With respect to the subject invention, the wedge shaped heat pipes occupy space which is not conveniently used for the field windings. These areas can only be filled with windings having expensive tapered geometries. Any reduction in field winding space increases the total heat generated in the field winding and, therefore, the heat load on the heat pipe itself. Additionally, due to the configuration of the heat pipe, only a limited surface area is utilized for the transfer of heat from the field pole windings to the coolant; and, consequently, such a heat pipe only realizes a limited cooling of the field pole windings.
Clearly, there is a need for a means for reliably cooling the rotor field windings of synchronous electric motors which may be readily maintained without the destruction of the rotor field windings and which provides an extended heat transfer surface for transferring heat from the rotor field windings to a heat transfer medium in an efficient manner.